The present application concerns fail-safe power supplies and, more particularly, a novel fail-safe power supply for providing opposite-polarity potentials to a semiconductor device, such as a GaAsFET device in a microwave amplifier.
It is known that some semiconductor devices require potentials of opposite polarity for operation. Several of these devices may be severely damaged if one potential is applied without application of the other potential. In particular, it is known that low noise amplifiers in the VHF, UHF and microwave regions of the electromagnetic spectrum can be realized utilizing field-effect transistors fabricated of gallium-arsenide semiconductor material. Such devices are presently utilized in at least the initial stages of satellite and ground-station receivers, as well as in terrestial microwave link receivers, television tuners and the like. These GaAsFET devices require that a negative gate-source potential always be present whenever a positive drain-source potential (for a N-channel device) is present, to prevent destruction of the device.
The prior art has used operator-controlled manually-activated arrangements which apply the negative gate potential prior to application of positive drain potential, and remove positive drain potential prior to removal of negative gate potential. Several circuits are known which provide a short circuit for the drain electrode potential if the gate electrode potential is not applied prior to application of the drain potential, with the short circuit causing a circuit breaker or fusible element to subsequently provide an open circuit between the device drain electrode and the positive potential supply. These protect circuits generally require a relatively large number of active devices and all merely interface some form of sequential switching arrangement between the device and a pair of opposite polarity power supplies which are assumed to exist. Accordingly, it is highly desirable to provide apparatus which will not only provide the opposite-polarity potentials, but which will provide such potentials in a manner such that a controlled (drain) potential can not be applied to the device unless a first (gate) potential is present.